At present, to improve the performance and economic characteristics of electrical machines as well as their serviceability, in the electrical engineering of highly developed countries there is a strong trend towards the increase of power in a single assembly.
The increase of power in an electrical machine is achieved either by intensified cooling of active parts of the machine (stator windings, rotor windings, etc.) making it possible to increase current density in the windings and to improve heat removal from the heated parts, or by enlarging the size of electrical machines.
At present, to increase the output of electrical machines, there is a tendency to intensify cooling of its active parts, which is economically most advantageous.
In large-sized electrical machines with superconducting field windings, the efficiency of operation is dependent in many respects on the cooling system of the winding.
Known in the art is a rotor of a cryogenic electrical machine, comprising a solid support structure formed as a massive carrying cylinder having in its outer surface slots and an U-shaped electrical insulation, and a superconducting field winding consisting of separate coils laid into the slots of the carrying cylinder and retained therein by a bandage cylinder.
The cooling system of the winding comprises axial passages to distribute the coolant around the periphery of the carrying cylinder and radial passages communicating with the axial passages through openings in the outer surface of the partitions between the slots of the carrying cylinder. The axial passages are formed in the partitions between the slots of the carrying cylinder, and the radial passages are formed in the inside of the U-shaped slot insulation and are interconnected along the slot bottom (Federal Republic of Germany application No. [2,511,104], 1976, Int. Cl. H02K 9/19).
The coolant supplied through the axial feeding passage passes through the openings in the outside of the partition into the radial passages of the U-shaped insulation and then flows through the passage portions in the slot bottom into the radial passages in the opposite side of the slot insulation wherefrom through the opening in the outside of the partition it is fed into the axial passage for discharging the coolant from the winding.
However, local heat evolutions in the superconducting field winding of the above rotor may bring about conditions under which caused by the separation of the coolant vapour-liquid mixture due to centrifugal forces is the accumulation of a vapour phase resulting in passage blocking which affects cooling conditions and decreases current-carrying capacity of the superconductor.
Keeping the passage from being blocked by increasing pressure at the point of coolant supply results in its temperature rise and, besides, requires additional means to be provided to raise the pressure of the coolant at the point of its supply.
In case of non-uniform heat generation along the rotor, locks may be formed in some but not all of the passages, which results in redistribution of the coolant flow rate along other passages, this phenomenon being irreversible. Besides, forming the axial passages in the partitions of the carrying cylinder makes it necessary to thicken the partitions, which results in poor utilization of the rotor active zone.
Formation of vapour locks blocking cooling passages of the superconducting field winding is eliminated in the rotor of a cryogenic electrical machine selected as the prototype of the present invention. The rotor comprises a superconducting field winding consisting of racetrack-shaped flat coils each of which is enclosed in a housing of a rectangular shape whose inner surface is machined to fit around the outer surface of the coil in close tolerance and which is completed with interconnected tangential and axial passages. Each coil is squeezed on its side surfaces between metal partitions having radial passages formed therein within the limits of the straight portion of the coil and communicating the central coolant-filled cavity of the rotor with the tangential passages of the housing. Besides, formed in the metal partitions are axial passages to discharge the coolant from the central cavity of the rotor. The electric insulation of the coil is formed by a layer of fiber glass provided on the inside and outside thereof and by an electrically insulating film provided on the side portions of the coil. The entire winding assembled with the housings and metal partitions is embraced by a metal bandage. ("Component Development for a 20-MBA Superconducting Generator" WELC, Moscow, 1977, paper 1.39).
The coolant flows along the passages in the following way. The liquid coolant flows through the axial passages provided in the outer surface of the housing from the end of the field coil, into the tangential passages of the housing wherefrom it is fed into the radial passages formed in the metal partitions and then into the central cavity of the rotor. The vapour phase of the coolant is removed from the central cavity of the rotor through the axial passages made in the metal partitions.
However, there occurs an indirect cooling of the superconductor through the insulation layer in said rotor, which affects the conditions of cooling of the superconducting field winding and hence decreases current carrying capacity of the superconductor.
Besides, each coil should be wound, impregnated and potted separately from the rotor body, whereupon a high-precision machining of the outer surface of the coil is required to obtain an appropriate shape and tolerances between coils and their housings. When assembling the winding, each coil is inserted into its housing, after which the coils with their housings are separated by metal partitions and held together by bolts. In such construction of the winding, the coil is held in the housing due to a precise machining of the mated surfaces of the housing and coil, which appreciably complicates the process of manufacture. Inaccuracy of machining may affect the solidity of the winding, cause displacements of the coils with respect to the partitions and housings and, hence, result in friction losses promoting a local temperature rise of the superconductor and decrease of its current carrying capacity.
It should be noted that forming of radial passages in the metal partitions result in the increase of the partition thickness, which affects utilization of the rotor active zone.